Generally, when aberration of an optical system must be limited to a certain value or lower, the optical system is provided with a diaphragm such that the aperture diameter of the diaphragm is adjusted to make the optical system brighter or to improve the resolution of the optical system. Also, when a plurality of beams are handled, a diaphragm is provided at a position at which a principal ray of the plurality of beams intersect with each other, i.e., a cross-over position in a primary optical system.
As described above, while a diaphragm is provided at a cross-over position in a primary optical system for handling a plurality of beams, problems described below arise if the diaphragm is provided in a secondary optical system. Specifically, since the cross-over position cannot be previously predicted in the secondary optical system, a problem arises in that the diaphragm cannot be matched with the cross-over position unless an adjuster lens is provided for adjusting the cross-over position to match the position of the diaphragm. Another problem is experienced when the diaphragm does not match the cross-over position, in which case some of a plurality of secondary electron beams closer to the optical axis are blocked by the diaphragm, so that even though the diaphragm is used to limit aberration of the optical system, the aberration cannot be reduced, resulting in a lower secondary beam detection efficiency and inability to eliminate cross-talk with adjacent beams.
As such, for providing a diaphragm in the secondary optical system, it is necessary to additionally provide a lens for focusing a cross-over image on the diaphragm, and an adjustor lens such as a two-stage lens for adjusting the dimensions of the cross-over at the diaphragm. However, an adjustor lens, if provided, will result in a longer optical system and require an aligner for the adjustor lens. A further problem arises as to the need for an aligner for the diaphragm, and an astigmatism adjuster for the cross-over, resulting in a complicated configuration of the diaphragm, a larger size of the overall apparatus, a longer time needed for adjustments, and a higher cost.
Thus, the utilization of a plurality of electron beams, i.e., multiple beams has been proposed for testing a mask pattern or a wafer for defects of LSI patterns and the like thereon at a high throughput. For example, a technology has been proposed for irradiating a plurality of regions on an object under testing with respective electron beams in order to improve throughput in such a defect detection. Also, another proposition has been made to the use of a field emission cathode which is capable of producing a large electron beam current at a low voltage when a fine pattern on the order of 0.1 micron is tested for defects using a low-energy electron beam.
However, when an array of field emission cathodes, which are inherently instable in operation, is used for an electron gun in a defect detection apparatus to generate multiple beams, even one field emission cathode in the array incapable of emission would cause the apparatus itself to fail to operate, possibly resulting in a significantly reduced operating rate.
Also, the instable operation of the field emission cathodes as mentioned above causes problems of difficulties in identifying fluctuations in emissions from the field emission cathodes and signals, and particularly, difficulties in providing an image with a large signal-to-noise ratio due to large shot noise.
In addition, a conventional electron gun of the multi-beam type electron beam apparatus has the following problems. FIG. 1 is a vertical sectional view schematically illustrating an exemplary conventional electron gun 100. An insulating ceramics 108 is supported within a cylindrical electron gun body 106. A bottom face of a ceramic seat 109 is fixed on a top face of the insulating ceramics 108. A single emitter 101 is fixed on a top surface of the ceramic seat 109 such that it is heated by a heater 3 which is a heating means. A lead for the heater 103, and high voltage cables 107 for a cathode extend from a bottom face of the insulating ceramics 108.
A Wehnelt member, i.e., Wehnelt electrode 102 is fitted over the cylindrical electron gun body 106. The Wehnelt electrode 102 has one end (upper end in the figure) integrally formed with an end wall which is provided with a single small hole (Wehnelt hole) 110.
The Wehnelt electrode 102 is fixed by a stop ring 104 at a position at which its end wall section is in close proximity to the emitter 101. The insulating ceramics 108 can be finely adjusted in its position in the horizontal direction by a plurality of finely movable screws 105 which extend through a peripheral wall of the electron gun body 106. Through the adjustment, the emitter 101 supported by the ceramic seat 109 on the insulating ceramics 108 is brought into alignment with the hole 110 provided through the end wall of the Wehnelt electrode 102.
However, there are problems in applying the method of finely adjusting the relative position between the single emitter 101 and Wehnelt electrode 102 as mentioned to an electron gun which comprises a multi-emitter having a plurality of emitters.
First, when the insulating ceramics is finely moved to finely adjust the multi-emitter in its in-plane position through the ceramic seat, the multi-emitter can be inclined. While the inclination does not constitute a grave problem for the single emitter, the inclined multi-emitter would result in different distances between the Wehnelt electrode and all emitters, and accordingly inconsistent emissions of electrons.
A second problem, which is also true for the single emitter, is that when the entire Wehnelt electrode is moved to adjust the axial distance between the Wehnelt electrode and multi-emitter, the Wehnelt electrode can be bumped against the multi-emitter which could thereby be broken.